A switch-mode converter having such an actuation circuit is described in DE 197 25 842 A1, for example. The switch-mode converter described in this publication is used as a power factor correction circuit (Power Factor Controller, PCF), that is to say is used to convert an AC voltage into a DC voltage with a drawn current which is proportional to that of the AC voltage.
An example of an actuation circuit for a switch controlling the power consumption in a PFC circuit is the integrated chip of type TDA4863 from Infineon Technologies AG, Munich, which is described in “Boost Controller TDA 4683, Power Factor Controller IC for High Power and Low THD”, Data Sheet, V 1.0, Infineon Technologies AG, May 2003. The application of this integrated chip in a power factor correction circuit is described in “TDA—Technical Description AN-PFC-TDA 4863-1”, Application Note, V1.2, Infineon Technologies AG, October 2003. A further actuation circuit for a switch in a step-up converter used in a PFC circuit is the integrated chip TDA 16888 from Infineon Technologies AG, Munich, which is described in the data sheet TDA 16888, 2000-02-28, Infineon Technologies AG, Munich.
On account of the demanded proportionality between the input voltage and the input current, the waveform of the power consumption in power factor correction circuits follows the waveform of the square of the input voltage. With a sinusoidal input voltage, the waveform of the power consumption is in sine squared form. This sine-squared power consumption at the input of the PFC circuit is contrasted by a constant drawn power at the output of the switch-mode converter when there is a constant load. In this context, a capacitor provided at the output of the PFC circuit serves as a buffer store which equalizes the difference between the time-variable power consumption at the input and the constant drawn power at the output and which ensures that the ripple in the power consumption causes only a slight ripple in the output voltage. This capacitor is a significant cost factor and therefore needs to be proportioned to be as small as possible, but this results in increased ripple in the output voltage.
The DC voltage generated by the switch-mode converter needs to assume a prescribed nominal value regardless of the load to which the DC voltage is supplied. A discrepancy between the output voltage and the nominal value appears when the average power transmitted differs from the power drawn by the load. So as always to be able to match the average power consumption to the drawn power, switch-mode converters have, in adequately known fashion, a voltage control loop or power control loop with a feedback loop via which information about the present value of the output voltage is fed back to the actuation circuit in order to be able to adjust the actuation of the switch controlling the power consumption in the event of load changes and an associated change in the output voltage. This feedback loop generates an error signal which is a measure of the discrepancy between the present value of the output voltage and the nominal value.
In principle, PFC circuits are subject to the requirement that the output voltage or the power consumption needs to be corrected as quickly as possible when there are changes in the load connected to the output, but the output voltage ripple existing for the reasons explained above must not adversely affect control.
To reduce any influence by the output voltage ripple on the control response, the feedback loop normally contains a control amplifier with an integrating response. This amplifier provides a control signal in which the error signal is integrated, so that cyclically recurring fluctuations in the error signal have negligible influence on the value of the control signal. However, the integration of the error signal means that such a system reacts slowly to sudden load changes, i.e. to abrupt changes in the load connected to the output. A sudden drop in the output voltage when there is an increase in the power drawn by the load can be regarded, from the point of view of the risk of damage to the switch-mode converter, as less critical than a sudden rise in the voltage when there is a decrease in the power drawn by the load, known as load shedding.
To avoid destroying the PFC circuit in the event of load shedding, actuation circuits for switches in PFC circuits are known which turn off the PFC circuit upon detection of an overvoltage. Hence, in the actuation chip TDA16888 explained above, a function is provided which linearly limits the power consumption of the switch-mode converter when the voltage applied to the output is 10% above the nominal value. If the voltage applied to the output is 20% above the nominal value, the switch-mode converter is immediately turned off and is not turned on again until the output voltage has fallen to the extent that it is now only 10% above the nominal value.
DE 44 22 066 C1 describes a circuit arrangement for limiting the output voltage of a clocked voltage controller in which an increase in the power consumption by the integrating control amplifier is evaluated as an indication of a load current having arisen in order to limit the switch-mode controller linearly without delay upon detection of such a sudden load change.
U.S. Pat. No. 5,581,450 and U.S. Pat. No. 5,619,405 respectively disclose switch-mode converters in which the gain of a control amplifier providing a control signal which is dependent on the output voltage assumes a first or a second gain value, depending on the extent to which the output voltage from the switch-mode converter differs from a nominal value, in order to be able to react quickly to sudden load changes as a result.
U.S. Pat. No. 5,502,370 and U.S. Pat. No. 5,565,761 respectively describe switch-mode converters in which the gain of an OTA (Operational Transconductance Amplifier) provided in the voltage feedback loop has a gain which is dependent on an output current from the switch-mode converter.
In addition, U.S. Pat. No. 6,140,777 fundamentally discloses the practice of producing the voltage control loop in a switch-mode converter in digital form using a digital signal processor (DSP) or a microcontroller.